The work in progress is directed towards the evaluation of the effects of acute and sustained methadone ingestion on the brain. The specific objectives are: (1) To measure relative responsiveness of the brain before, during sustained drug intake, and during withdrawal using sensory evoked potentials recording from cortical and subcortical electrodes permanently implanted in critical brain sites in monkeys. (2) To investigate the possible site of action of the drug in the brain by noting which systems or areas are most affected by changes in cerebral electrophysiology of the animals. (3) To note the effects of the drug on the animal's behavioral efficiency and on its sleep patterns. (4) To study the interaction of methadone with other drugs commonly used by the addict. (5) To quantify and describe irreversible changes in electrophysiological and behavioral measures if these occur.